Adult male residents of 13 counties of western Washington state in whom testicular cancer had been diagnosed during 1977-1983 (n = 333) were interviewed over the telephone regarding their history of genital tract conditions, including vasectomy. For comparison, the same interview was given to a sample of 729 men selected from the population of these counties by dialing telephone numbers at random. A higher proportion of cases than controls reported having had a vasectomy (relative risk = 1.5, 95 per cent confidence interval = 1.0-2.2). However, the association was restricted entirely to Catholic men. Whereas a history of vasectomy was reported with approximately equal frequency by Catholic and non-Catholic cases, only 6.3 per cent of Catholic controls reported such a history in contrast to 19.7 per cent of other controls. While the authors cannot rule out the possibility that there is a true difference of the effect of vasectomy on the incidence of testicular cancer as a function of religion, it seems more plausible that selective underreporting by Catholic controls has produced a spurious relation.228 adult men resident of 13 western counties in Washington state, who had testicular cancer diagnosed between 1977-1983, were interviewed by telephone regarding vasectomy and other genitourinary history. 513 controls were selected at random from households chosen from the telephone directory by the Waksberg method. Only married men, aged 20-69, without history of cryptorchidism were included. Cancer patients and controls did not differ significantly in education, religion, smoking, coffee use or state residence. There was a small excess risk among men who reported history of inguinal hernia and testicular injury, and among those diagnosed with varicocele. Vasectomy was reported by 22.1% of the ever-married cases and 17.2% of controls. The relative risk of testicular cancer was the same for all groups except Catholics, whose risk was 8.7 times that of Catholic controls. There are no similar associations in the literature, either between testicular cancer and vasectomy, or between religion and under-reporting of contraceptive use, to explain this apparent under-reporting of vasectomy by Catholic controls.